如何从模式堆栈中删除特定页面？

Question

我有四页页面 1-page 2-page 3-page 4.我使用推送模式异步进行向前导航。当我在第 4 页中点击按钮时，它正在导航到第 2 页。但是在第 2 页点击后退按钮会显示堆栈页面的所有历史记录。那么如何从模态堆栈中删除页面。我也使用了 navigation.Remove(page) 但它引发了异常。请指导。

foreach (var page in Navigation.ModalStack)
{
    if (page is Page3)
    {
        await PopModalPage();
    }
}

foreach (var page in Navigation.ModalStack)
{
    if (page is Page4)
    {
        await PopModalPage();
    }
}

要从第 4 页导航到第 2 页，我正在使用此代码 还有比这更优化的方法吗？？

Answer 1

您可以使用以下辅助函数

/// <summary>
/// Class with extension methods for INavigation interface to help working with the modal page stack
/// </summary>
public static class ModalHelper
{
    /// <summary>
    /// Unwinds the modal stack of the navigation until reaching a page with the given type
    /// </summary>
    /// <typeparam name="PageType"></typeparam>
    /// <param name="navigation">The navigation object of the current top page</param>
    /// <returns></returns>
    public async static Task UnwindModalStackTo<PageType>(this INavigation navigation) where PageType : Page
    {
        await navigation.UnwindModalStackTo(p => p is PageType);
    }

    /// <summary>
    /// Unwinds the modal stack of the navigation until reaching the given page 
    /// </summary>
    /// <param name="navigation">The navigation object of the current top page</param>
    /// <param name="page">The page where to stop unwinding the modal stack</param>
    public async static Task UnwindModalStackTo(this INavigation navigation, Page page) 
    {
        await navigation.UnwindModalStackTo(p => p == page);
    }

    /// <summary>
    /// Unwinds the modal stack of the navigation until reaching a page that fulfils the predicate
    /// </summary>
    /// <param name="navigation">The navigation object of the current top page</param>
    /// <param name="predicate">A function which tests whether to stop at a given page</param>
    public async static Task UnwindModalStackTo(this INavigation navigation, Func<Page, bool> predicate)
    {
        bool found = false;

        while (navigation != null && navigation.ModalStack.Count > 0)
        {
            // Get the current top page of the modal stack
            Page topPage = navigation.ModalStack[navigation.ModalStack.Count - 1];

            // Get the second page in the modal stack from the top (This one will become top next after we pop)
            Page parentPage;
            if (navigation.ModalStack.Count > 1)
            {
                parentPage = navigation.ModalStack[navigation.ModalStack.Count - 2];
            }
            else
            {
                parentPage = null;
            }

            // When the top page fulfills the predicate, stop
            if (predicate(topPage))
            {
                found = true;
                break;
            }

            // Pop the top page
            await navigation.PopModalAsync();

            // We need to use the navigation of the new top page from here on
            navigation = parentPage?.Navigation;
        }

        // When the target page was not found, throw an exception
        if (!found)
        {
            throw new Exception("Desired page not found in modal stack");
        }
    }

}

请注意，在我们从堆栈中弹出顶部模式页面后，我们必须使用来自新顶部页面的导航才能继续。

---

例子：

假设我们有一个包含五个页面的应用程序：MainPage、Page1、Page2、Page3、Page4。

Mainpage 有一个按钮可以打开 Page1 modal，Page1 有一个按钮可以打开 Page2 modal，以此类推。

在主页中：

private async void Button_Clicked(object sender, EventArgs e)
{
    Page p = new Page1();
    await Navigation.PushModalAsync(p);
}

在第 1 页：

private async void Button_Clicked(object sender, EventArgs e)
{
    Page p = new Page2();
    await Navigation.PushModalAsync(p);
}

等等……

现在在 Page4 中（为了返回到 Page2）我们使用以下代码关闭所有打开的模式页面，直到到达 Page2。

private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.UnwindModalStackTo<Page2>();
}

Answer 2

RemovePage 仅支持从NavigationStack 中删除指定页面。

像这样：Navigation.RemovePage(Navigation.NavigationStack[Navigation.NavigationStack.Count - 1]);

您可以参考这个以获得更多详细信息： https://docs.microsoft.com/en-us/dotnet/api/xamarin.forms.inavigation?view=xamarin-forms

Answer 3

如果您想返回 2 页，这将起作用

        Navigation.RemovePage(Navigation.NavigationStack[Navigation.NavigationStack.Count - 2]);
        Navigation.RemovePage(this);

首先我们删除当前页面之前的页面，然后是当前页面。

Answer 4

请使用await Navigation.PopModalAsync() 并通读this link 以获取有关ModalPages 的信息。

编辑：

尚未对其进行测试，但可以帮助理解该方法：

for (var i = Navigation.NavigationStack.Count - 1; i > 0; i--)
{
    if (Navigation.NavigationStack[i] is Page2)
    {
        return;
    }
    await Navigation.PopModalAsync();
}